Modern telecommunications has been revolutionized by the extremely high communications bandwidth of fiber optics technology and transmission systems. Vast amounts of information equaling hundreds of thousands of phone calls or hundreds of video channels can be carried on a single beam of modulated laser light. Bandwidth capabilities have been more than doubling every two to three years.
A fiber optic transmission system typically includes an optical transmitter, an optical fiber, an optical amplifier, and an optical receiver.
The optical transmitter receives an electrical digital signal and converts it into an optical signal by modulating a laser light into optical signal pulses that represent the various values or states of the electrical digital signal.
The optical signal pulses are transmitted through the optical fiber and, generally, are amplified by one or more optical amplifiers before being converted back into electrical digital signals by the optical receiver. This is generally referred to as the optical link or optical channel.
The optical signal pulses arriving at the optical receiver must be of sufficient quality to allow the optical receiver to clearly distinguish the on-and-off pulses of light signals sent by the optical transmitter. However, noise, attenuation, and dispersion are a few of the impairments that can distort the optical signal pulses, rendering the optical signal pulses marginal or unusable at the optical receiver and making it difficult or impossible to accurately detect or reconstitute the electrical digital signal. This distortion nonuniformly broadens, spreads, or widens the various optical signal pulses, reducing the spacing between the pulses or causing them to overlap, thereby rendering them virtually indistinguishable.
Conventionally, a properly designed optical channel can maintain a Bit Error Rate (“BER”) of 10−13 or better. When an optical channel degrades to a BER of 10−8, a telecommunications system may automatically switch to an alternate optical channel in an attempt to improve the BER. Otherwise, the telecommunications system must operate at a reduced or lowered bandwidth, with poorer overall system performance.
Dispersion is a major contributor to distortion of optical signal pulses, leading to increases in the BER. The distortion caused by dispersion generally increases with increases in the bandwidth or data rate, and with increases in the optical fiber transmission distance.
Dispersion has generally been identified as being caused by (1) chromatic dispersion, or (2) polarization mode dispersion (“PMD”).
Chromatic dispersion occurs when the various frequency components, or colors, of the optical signal pulse travel at different speeds through the optical fiber and arrive at the optical receiver at different times. This occurs because the index of refraction of a material, such as the optical fiber, varies with frequency or wavelength. As a result, the optical signal pulses are distorted through chromatic frequency-related pulse spreading.
Some of the major solutions for chromatic dispersion have included: (1) single-mode propagation, (2) Distributed Feedback (“DFB”) lasers with narrow output spectra, and (3) low attenuation/modified-dispersion optical fibers. All of these advances have contributed to increased bandwidth by allowing the optical signal pulses to pass through the optical fiber with relatively low or reduced dispersion, and hence, relatively low or reduced optical signal distortion.
Single-mode propagation (or use of narrow wavelengths) was achieved through the development of single-mode optical fiber. This optical fiber allows only a single mode of light to propagate through the optical fiber. The DFB laser provides a light source to use with single-mode optical fibers. The DFB laser produces a light with an extremely narrow distribution of output frequencies and wavelengths, minimizing the chromatic dispersion problem. The low attenuation/modified-dispersion optical fiber provides a dispersion-shifted optical fiber that minimizes the speed-vs-wavelength dependency at a specific wavelength.
Previously, chromatic dispersion received greater attention because its adverse effects were initially more limiting at prior, lower available bandwidths and data rates. Now, PMD receives considerable attention due to its potential limitation on optical transparent high-speed long-distance light wave systems, as well as on multi-channel cable television transmission systems.
PMD refers to distortions in the two orthogonal light wave components of the polarized light signal pulses emitted by the optical transmitter. In an ideal optical fiber, which has a perfectly circular cross-section and is free from external stresses, the propagation properties of the two polarized light signal components are identical. However, imperfections introduced in the manufacturing process may result in an optical fiber that is not perfectly circular. In addition, an optical fiber that has been installed may suffer from external stresses such as pinching or bending. These manufacturing imperfections and external stresses cause the two polarization components of the polarized light pulses to have different propagation characteristics, which in turn give rise to PMD.
Despite the manufacturing-induced imperfections, optical fibers (for each optical frequency ω) have two input states (“principal states of polarization”, or “PSP's”) in which a matching light pulse will undergo no PMD spreading. However, light pulses can be input into a fiber in an arbitrary state, and this leads to the pulses being split into two components that propagate independently through the fiber at different velocities. When these components reach the end of the fiber they recombine as two sub-pulses split in time. The delay between the two sub-pulses is designated as the differential group delay (“DGD”), τ.
The DGD and the PSP's of a long fiber are not only dependent on the wavelength or frequency of the optical pulses, but they also fluctuate in time as a result of environmental variations such as temperature changes, external mechanical constraints, and so forth. Their behavior is random, both as a function of wavelength at a given time and as a function of time at a given wavelength.
Various techniques have been proposed and are known for compensating for PMD in optical transmission systems. Unfortunately, little has been done to harness the power and potential of such PMD compensation technologies to solve problems and afford additional functionalities beyond correction and compensation. In other words, PMD has only been seen as a problem to be corrected or eliminated, and accordingly, opportunities to exploit PMD rather than just compensate for it have not even been considered by those skilled in the art.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.